Heating and cooking food with microwave energy in a microwave oven is quite commonplace. However, although the molecular friction resulting from the high frequency electromagnetic radiation cooks the food adequately, it does not brown the cooked food. This negatively affects the taste, visual appeal, and general gastronomic appeal of the food. It is highly desirable when cooking food with microwave energy to brown the exterior of the food so the food resembles food cooked by conventional methods. (Throughout this specification, the term "brown" includes browning, crisping, searing, and otherwise heating the exterior surface of food.) Therefore, special provision must be made to brown the exterior of the food and to avoid undercooking the exterior of the food due to the surface cooling effect on the food. Many products, devices, and methods have been devised in the prior art for browning, searing, and otherwise heating the exterior surface of food cooked in microwave ovens.
An increasingly common method for browning food has been to incorporate microwave interactive layers into disposable laminate materials used to cook the food. Such laminates are characterized by their ability to absorb microwave energy and convert it to heat which may be conductively and radiantly transmitted to the food. Semiconductive materials, ferro-magnetic metals, metal oxides, and thin elemental metals are some of the materials used to form the microwave interactive layer. U.S. Pat. No. 4,230,924 to Brastad et al. and U.S. Pat. No. 4,267,420 to Brastad disclose using a flexible wrapping material for browning food in a microwave oven including a thin aluminum layer sandwiched between a layer of polyester and a layer of polyethylene. Laminates of the type disclosed in the '924 and '420 patents suffer numerous drawbacks: the sheet laminate may not be sufficiently flexible to contact the exterior surface of the food without extending away from the food at some portions; where the exterior surface of the food is irregular parts of the laminate do not contact the food and the food is not adequately browned; and portions of the laminate may actually melt, shrivel, burn, or otherwise disintegrate when subject to microwave energy. Additionally, these patents do not disclose a scheme for forming receptacles or packages out of a long web suitable for use in modern, high speed package forming equipment.
Many of the problems noted with respect to the flexible wrapping material of the '420 and '924 patents were overcome by U.S. Pat. No. 4,641,005 to Seiferth. Seiferth discloses the concept of bonding a thin continuous layer of interactive metal formed on the smooth surface of a heat stable thin layer of plastic such as polyester directly to a dimensionally stabilizing layer of microwave transparent stock material such as paper. The dimensionally stabilizing layer imparts a number of additional advantages not the least of which is to serve as the integral part of a package suitable for shipping, displaying, cooking, and serving food. In addition, the process of bonding the interactive metal-coated side of the plastic layer is also believed to impart a limiting temperature characteristic to the dimensionally stabilized laminate. This provides rigidity and support to the metallized plastic layer and thus prevents contortion and irregular heating. The resulting laminate is formed with sufficient flexibility to permit the laminate to be "wrapped" around a substantial portion of the contour of a food item to be cooked.
Despite the breakthrough provided by the '005 patent, or perhaps because of it, numerous concerns relating to the formation of disposable food packages for microwave cooking have arisen. These concerns relate to the formation of the food container in such a way as to provide a low cost yet effective mechanism through which a disposable microwave cooking container may be manufactured and the food articles may be encased in the container. In particular, experience with microwave interactive laminates has shown that, under certain circumstances, food may be lost during the packaging process unless it can be encased rapidly and efficiently in an automated process. Another problem has been to provide graphics, sealing material and the microwave interactive layer so that each of these important parts of a food package are in proper registry with each other. Still another concern is to facilitate handling the package in and out of a microwave oven. Finally, formation of the package so the microwave interactive layer does not damage the seal during cooking in a microwave oven can be a vexing problem in certain circumstances.
U.S. Pat. No. 4,034,973 to Hams discloses an automated in-line mailing system in which a continuous web of paper is supplied, cut, folded, collected, and inserted into envelopes. This process is performed automatically and is controlled by a column of marked indicia on the paper web. While this patent is believed to be representative of automated systems controlled by indicia, the Hams system is not directed to food packaging. It does not even hint at the problems which plague the mass production of food containers for heating and browning food in a microwave oven.
Although U.S. Pat. No. 4,735,513 to Watkins et al. discloses forming a microwave receptacle from a long web of material, it too does not address the above problems. Performing various functions such as printing graphics, applying sealant, and applying microwave interactive material in registry on the web are not disclosed.
No one has disclosed a packaging material or a method of forming a packaging material that addresses and solves all of the discussed problems.